Generally speaking, a construction machine such as a hydraulic excavator is equipped with a hydraulic pump driven by a prime mover, a hydraulic actuator, and flow control valves controlling the supply and discharge of the hydraulic working fluid with respect to the hydraulic actuator. Each flow control valve has a meter-in restrictor and a meter-out restrictor. The meter-in restrictor controls the flow rate of the hydraulic working fluid flowing into the hydraulic actuator from a pump, and the meter-out restrictor controls the flow rate of the hydraulic working fluid discharged from the hydraulic actuator to a hydraulic working fluid tank. Examples of the hydraulic actuator in a hydraulic excavator include a boom cylinder driving a boom and an arm cylinder driving an arm.
In a construction machine equipped with such a hydraulic actuator, it can occur that the weight of the support object of the hydraulic actuator (which, in the case, for example, of an arm cylinder, includes an arm and a bucket (attachment) acts as a load in the same direction as the operational direction of the hydraulic actuator (hereinafter also referred to as the “negative load”). In this case, the operational speed of the hydraulic actuator increases, and there is a shortage of hydraulic working fluid flow rate on the meter-in side, so that there is a fear of generation of a breathing phenomenon (cavitation). The breathing phenomenon may lead to deterioration in the operability of the construction machine and to damage of the hydraulic apparatus.
To solve the above problem, there is known a construction in which a meter-out control valve is provided in a meter-out passage leading to a hydraulic working fluid tank from a hydraulic actuator and in which the opening area of the meter-out control valve is adjusted in accordance with a cylinder pressure, whereby the cylinder speed is suppressed and a breathing phenomenon is prevented (See, for example, Patent Document 1).